The phrase "party opposite" refers to someone who is on the other side or opposing side of a situation or argument. It is commonly used in legal or political contexts to describe the party or person who disagrees with you. Full definition
«There is not a single credible political party proposing more spending apart from the Labour party opposite.» (politics.co.uk)
We know that the party opposite is perfectly comfortable with wealthier Canadians paying a lower tax rate than middle - class Canadians. (canadianbusiness.com)
He told MPs: «Despite the disappointment of the party opposite that supports irresponsible and damaging strikes, it looks like something of a damp squib.» (politics.co.uk)